While many challenges remain for hydrogen vehicles, in recent years researchers have made big improvements in the oft-maligned technology, including greatly lowering its cost. As a result, fuel-cell vehicles look poised to play a significant role in meeting ambitious vehicle emissions regulations, particularly in places such as California.

"GM, Toyota, and a couple of other automakers have done a lot of great work. Fuel cells are getting close to being viable, closer than most people might think," says Brett Smith, co-director for manufacturing, engineering, and technology at the Center for Automotive Research, a nonprofit in Ann Arbor, Michigan.